It's just me or does any of you guys has a Plasma Defender using ammo called 'MCA'? Which, of course, I couldn't find a single one in the game.

Its just you,
The plasma defender uses energy cells. Your probably reading the abbreviation for the Max Charge variant, which would show up as MC on your HUD to indicate the type of ammo variant your using.

MCA stands for "magic companion ammo" I'm guessing some how you got a version that a companion uses?

It might be Gannons plasma defender, it uses ammo unique to him at least. You might get it when leaving casinos with Gannon as follower because of some bug but you cant get the ammo for it.

Oh, that's why! Yes, I got it from Gannon. If you can't use it, why does it even show up in his inventory anyways? Is it a bug? Well, since it is useless, I'll give it back to Gannon then. Thanks for your help!

The gun uses Mr Chris Avalone as ammo? Must be a big gun.